1. Field of the Invention
The present invention relates to a protective apparatus for an inverter which protects switching elements constituting the inverter against an overvoltage generated at a turn-off operation.
2. Prior Art
There is known a voltage type inverter composed of self-extinguishing type switching elements such as gate turn-off thyristors (GTOs). A snubber circuit is provided to each switching element constituting an inverter of this type in order to absorb a voltage surge generated by circuit inductance during a turn-off operation. The snubber circuit is composed mainly of a serial circuit of a capacitor and a forward-biased diode, connected in parallel with a switching element to be protected, with a discharge resistor connected in parallel with the diode for the discharge of the capacitor.
If a short-circuit between lines or a short-circuit to ground occurs at the load side of the inverter while the inverter constructed as above is operated by supplying power from a DC power supply connected in parallel with a high frequency absorbing capacitor, an overcurrent protective apparatus immediately starts operating to cause the inverter to turn off when the inverter output current Io reaches an overcurrent detection level Ioc, e.g., 130% of the rated output current. Since the interruption of an overcurrent is effected by a turn-off operation of switching elements, surge voltages are generated across the switching elements. A peak value Vp of the surge voltage is given by the following equation, assuming that DC power supply voltage is Vd, circuit stray inductance is Ls and capacitance of a snubber capacitor is Cs: ##EQU1##
The capacitance Cs of the snubber capacitor is so selected that the voltage peak value Vp obtained by the equation (1) does not exceed a voltage withstanding specific to each switching element.
In the case that a DC power supply has the characteristics that its voltage rises as a load is reduced, such as shown by the characteristic curve of FIG. 3 of current Id relative to voltage Vd of a fuel battery, turning off the switching elements which is carried out in order to interrupt the overcurrent generated at the inverter output side during a light load operation as seen from the DC power supply causes the generation of surge voltages across the switching elements. The peak voltage Vp of the surge voltage in such a case becomes higher than that at a heavy load operation by the amount of the increased fraction of the voltage Vd. The problem associated with voltage withstanding can be solved if switching elements having a voltage withstanding higher than such a surge voltage are used. However, this results in an increase in cost of the apparatus.
It can be considered that the capacitance Cs of a snubber circuit capacitor be made large. However, the charge energy increases so that an increase in loss at the snubber resistor becomes large during the discharge, to hence lower the efficiency of the inverter apparatus.
Alternatively, the level of overcurrent detection may be lowered to suppress the surge voltage. However, the number of interruptions of the operation of the inverter may increase undesirably even under a load within a normal range.